A recently developed vacuum tube for handling r.f. signals includes a cathode for emitting a linear electron beam, a grid positioned parallel and in close proximity to the cathode (no farther than the distance an emitted electron can reach in a quarter cycle of the signal being handled by the tube) for current modulating the beam, and a cavity resonant to the frequency of the signal positioned between the grid and a collector electrode for the beam. The grid is coupled by a structure resonant to the frequency being handled by the tube to an input of the tube. Very high efficiency is achieved with such a tube by biasing the grid so that current flowing from the cathode toward the grid occurs for no more than one half cycle of the r.f. signal handled by the tube. Typically, the bias voltage between the grid and cathode is very small or zero. To prevent emission from the grid, it is formed of a non-emissive material, such as pyrolytic graphite, or molybdenum coated with zirconium. As applied to the electron beam flowing beyond the grid, the terms "current-modulated," "space-charge modulated," "density-modulated" and "intensity-modulated" are synonymous, and refer to concentrations (or "bunches") alternating with depletions of particle density (or space-charge density) along the beam. Speeding and slowing of particle velocity is indicated by the term "velocity modulation."
In one prior art configuration, a resonant input circuit supplies electric fields in opposing phase between the cathode and grid and between the grid and an accelerating anode positioned between the grid and an output cavity. In another prior art device, a second resonance cavity positioned between the output cavity and the accelerating anode is adjusted so the resonance frequency thereof is above the frequency being handled by the tube, to increase the average efficiency of the tube. These prior art structures are disclosed in the commonly assigned U.S. Pat. Nos. 4,480,210, 4,527,091 and 4,611,149. Devices incorporating the teachings of at least some of these patents are commercially available from applicants' assignee under the registered trademark KLYSTRODE.
While the prior art tubes have performed admirably, they are rather large. One of the factors contributing to the size of the prior art tubes of the general type disclosed in said patents is the resonant structure for coupling an input signal to the cathode-grid assembly. In the past, the resonant structure for coupling the input signal to the cathode-grid assembly has included a resonant cavity coaxial with the cathode and the electron beam emitted from it. This resonant cavity has a length in the direction of the beam axis that is nominally either a half wavelength at the frequency handled by the tube or a full wavelength at this frequency. In practice, it is most usually the latter.
The input signal to the cavity is transformer-coupled to the cavity. In this document, the phrase "transformer coupled to the cavity" signifies that power coming into or going out of a coaxial cable is coupled by r.f. magnetic fields to the cavity via loop coupling or by r.f. electric fields via probe coupling.
A metal structure in the input resonant cavity couples the field established in the cavity in response to the input signal to the grid. An r.f. electric field is thereby established between the grid and cathode, to current-modulate the electron beam. An r.f. field is also established in opposing phase between the grid and anode. While the size constraints associated with the input resonant cavity are not an impediment to many commercial uses of the KLYSTRODE brand tube, it is a substantial detracting factor for many military and space applications.
To reduce the length of the resonant structure for coupling an input signal to the cathode-grid assembly, there is disclosed in the co-pending, commonly assigned application of Lien, Ser. No. 07/508,442, filed Apr. 13, 1990, entitled "Vacuum Tube Including Grid-Cathode Assembly With Resonant Slow-Wave Structure," filed concurrently herewith, an improved vacuum tube of the above type. In the co-pending application, an input signal excites an r.f. electric field in the region between the cathode and grid by means of a slow-wave structure that is approximately resonant to the frequency of the signal. By utilizing a slow-wave structure, rather than a coaxial, resonant cavity as in the prior art, the size of the tube is considerably reduced. The slow-wave structure is preferably incorporated in the grid assembly, either in a support structure for the grid or in a portion of the grid which applies an accelerating r.f. voltage to the beam, to current modulate the beam. The prior art tubes, as disclosed in the aforementioned patents, for example, are designed without anticipating that such a slow-wave resonant structure might be incorporated in the grid-cathode assembly.
In the prior art tubes, as disclosed in the aforementioned patents, regeneration and increased gain are obtained by energy transfer to a pre-bunched beam from an r.f. field in the grid-anode space. To achieve this regeneration and increased gain, a driver circuit for the prior art tubes becomes electrically quite complex and difficult to design. Considerable time and effort for empirical design of the driver circuit and tube are necessary to achieve the desired results. It is difficult to adjust the driver cavity and tube parameters to achieve the optimum relative intensity and phase relation of the electric fields in the two r.f.--field regions.
It is, therefore, an object of the present invention to provide a new and improved r.f. amplifying vacuum tube wherein an electron beam is bunched (or current-modulated) by a grid in proximity to an electron beam-emitting cathode and the beam bunching is enhanced through velocity-modulation effects induced by a tuned cavity downstream of the grid.
Another object of the present invention is to provide a new and improved amplifier tube for an r.f. signal wherein an electron beam is current-modulated with increased efficiency.
An additional object of the invention is to provide a new and improved, highly efficient r.f. amplifying tube having an electron beam that is current-modulated by a control-grid structure and velocity-modulated by a resonant cavity structure wherein the relative phase relation between both structures is easily controlled.
A further object of the invention is to provide a new and improved amplifier tube for r.f. signals wherein an electron beam is current- and velocity-modulated, which tube is particularly adapted for use in conjunction with a cathode-grid assembly including a resonant slow-wave structure, which enables the size of the tube to be reduced.
A further object of the invention is to provide a new and improved r.f. amplifying vacuum tube with an electron beam that is both current- and velocity-modulated, wherein the modulations are precisely controlled to achieve optimum regeneration and increased gain without incurring oscillatory instability.
A further object of the invention is to provide a new and improved r.f. amplifier vacuum tube wherein an electron beam is current- and velocity-modulated in response to an r.f. signal and circuitry is provided to assure that the current and velocity modulation processes reinforce with respect to gain and efficiency.
An additional object of the invention is to provide a new and improved r.f. vacuum tube with an electron beam that is current- and velocity-modulated, wherein relatively simple and easily designed driver circuitry is employed.
Still an additional object of the invention is to provide a new and improved r.f. vacuum tube with a current- and velocity-modulated electron beam, which tube is easily designed, while achieving optimum relative magnitude and phase of fields which control the two modulation processes.